Occasionally after falling into the void while in the End, and respawning. The Enderdragon's health bar will still show in the Overworld for a couple seconds.
MC-4474
Boss Health Bar Temporarily Viewable after Respawn and Loading Another World
Resolved
Fixed
9
Confirmed
boss-bar
Minecraft 1.4.4, Minecraft 1.4.5, Snapshot 12w49a, Snapshot 12w50b, Minecraft 1.4.6, ..., Minecraft 14w30b, Minecraft 14w30c, Minecraft 1.8, Minecraft 1.8.1, Minecraft 1.8.8
Minecraft 15w31a
Retrieved
Same with the Wither.